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How does a diesel engine ignite fuel without spark plugs?

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diesel engine and precombustion chamber — diesel engine Diesel engine equipped with a precombustion chamber.

A diesel engine ignites fuel through a process known as compression ignition, which differs fundamentally from the spark-ignition systems used in gasoline engines.12

Instead of relying on an electric spark to initiate combustion, a diesel engine compresses only air within the cylinder. These engines are designed with high compression ratios, typically ranging from 14:1 to 22:1. This intense compression heats the air to temperatures often exceeding 526 °C (979 °F).1

As the piston approaches the top of its stroke, fuel is injected directly into this highly compressed, hot air. Because the air temperature is higher than the fuel's "auto-ignition" temperature, the fuel spontaneously reacts with the oxygen and ignites upon contact. This method allows diesel engines to achieve high efficiency by avoiding the preignition limitations that affect high-compression spark-ignition engines.1